This invention relates to an envelope shape generator for tone signal control and, more particularly, to an improvement in a control for rapidly attenuating a generated envelope shape called "forcing damp". This invention relates also to an envelope shape generator capable of attenuating an envelope shape signal rapidly for restraining noise which is generated in a tone signal controlled by this envelope shape signal when the level of the envelope shape signal has decreased during attenuation.
An envelope shape for tone signal control generally consists of characteristics called A (attack), D (decay), S (sustain) and R (release). Among these characteristics, the release characteristic is one in which the level attenuates relatively gradually from a sustain level to zero level. Inclination of attenuation in this release characteristic generally becomes increasingly gradual with a lapse of time. This is a simulation of general attenuating characteristic of a natural musical tone and a tone being sounded is muted in a natural manner.
It is also known in the art to attenuate an envelope shape rapidly by making inclination of attenuation steeper from a certain point in the release. This is a control called "damp" or "forcing damp" which is performed by intentional operation of a damper operator or in a case where a tone which is being sounded needs to be muted rapidly for sounding of a new tone.
It is known to generate an envelope shape signal for tone signal control in the form of data in decibel representation (e.g., Japanese Patent Publication No. 7600/1980 and U.S.P. 4,267,763). This art is employed because it has the advantages that multiplication of data in linear representation can be replaced by a simple addition in data of decibel representation and that, if envelope shape data in decibel representation is converted to data in linear representation, its attenuating portion attenuates with an exponential function characteristic and this is acoustically desirable. If, however, an envelope shape in decibel representation as shown in FIG. 12a is converted to data in linear representation in this case, the characteristic of the converted data becomes as shown in FIG. 12b in which the attack portion lacks steepness in rising. Efforts have therefore been made in the art to ensure steepness in rising in the attack portion as shown in FIG. 12c.
The above mentioned "forcing damp" is disclosed in, for example, Japanese Preliminary Patent Publication 65489/1983. In performing forcing damp in generating an envelope shape data in decibel representation, the forcing damp portion in the prior art has the same uniform inclination characteristic as other attenuating portions though the inclination is steeper in the forcing damp portion than in the other portions (see FIG. 13a).
In performing forcing damp, there is requirement, on one hand, that the envelope shape should be attenuated as rapidly as possible (in other words, a preceding tone should be erased as rapidly as possible) and also there is requirement, on the other hand, that excessively rapid attenuation causes a click noise so that the envelope shape should be attenuated with an inclination which will not cause such click noise. As described above, if an envelope shape in decibel representation as shown in FIG. 13a is converted to data in linear representation, it exhibits an exponential function characteristic as shown in FIG. 13b. That is, the forcing damp portion attenuates with the exponential function characteristic as in a normal attenuating portion. For this reason, the inclination of the envelope shape is excessively steep at the beginning of forcing damp and becomes gradual towards the end of forcing damp. The excessively steep inclination at the beginning of forcing damp causes a click noise as described above. The inclination of the forcing damp portion in decibel representation therefore is restricted with a result that a period T of forcing damp tends to become unnecesarily long. Besides, since the forcing damp portion attenuates with an exponential function characteristic in the converted data in linear representation (FIG. 13b), it takes unnecessarily long time before the tone is extinguished completely with a result that starting of a next tone is delayed.
The known damp control is performed compulsorily regardless of the level of the envelope shape at that time only when necessity has arisen due to external factors such as operation of a damper operator and depression of a new key. If such necessity does not arise, the envelope shape attenuates in accordance with a normal release characteristic.
Since a digital tone waveshape signal generally is a quantized signal, a certain number of bits is necessary for each sampled value for effectively representing the tone waveshape. If, however, the amplitude of the digital tone waveshape signal is controlled in response to the envelope shape signal, the amplitude of the digital tone waveshape signal becomes small as the level of this envelope shape signal becomes small with a result that the bit number for representing each sampled value of its tone waveshape signal becomes small. Schematically illustrated, if the level of the envelope shape signal is relatively large and accordingly the amplitude of the digital tone waveshape signal is relatively large, the tone waveshape can be effectively represented as shown in FIG. 15a and S/N ratio is good in this case. If, however, the level of the envelope shape signal is small and accordingly the amplitude of the digital tone waveshape signal is smaller than a bit number capable of effectively representing the tone waveshape, the tone waveshape cannot be effectively represented as shown in FIG. 15b and, in this case, S/N ratio is deteriorated and a hissing noise occurs.